Keywords: atmospheric freeze drying, drying kinetics, HPD, heat pump drying, multimode heat input, physical properties, postharvest technology, vacuum freeze drying, vortex tubes
A novel atmospheric freeze-drying system using a vortex tube and multimode heat supply
This article presents results of an experimental study of (AFD); atmospheric freeze-drying which uses a vortex tube to produce low temperature air, as an alternative to vacuum freeze-drying (VFD). An experimental setup was designed and built to permit simultaneous application of convection, conduction and radiation heat input to the drying material without heating it above the freezing point of water contained in it. Potato slices were used as a model heat sensitive product for testing. Four heat input schemes were compared experimentally: Case 1 – pure convection, Case 2 – two-stage convection, Case 3 – radiation-coupled convection and Case 4 – radiation-conduction coupled with convection. Drying kinetics, quality parameters and physical properties of the model product (colour, rehydration properties, scanning electron micrographs) were analysed. Experiments were also conducted in a well-instrumented heat pump dryer and a vacuum freeze dryer for direct comparison with the AFD results. The proposed AFD system is shown to be a viable alternative to vacuum freeze-drying.